A century of Toxoplasma gondii research

Toxoplasma gondii (Fig. 1) is a protozoan parasite that can be transmitted directly from cats to humans through faecal contamination of food, or indirectly from cats to livestock and then to humans through undercooked meat. Around 30% of humans in the United Kingdom are infected, and as such, harbour dormant cysts in their brain, but few have overt symptoms of disease.

Neurological disease can occur in these people if they become immunosuppressed (Fig. 2). The possibility that apparently healthy people with infection are more likely to develop psychiatric disease, including schizophrenia and depression, is under investigation. Infection during pregnancy can cause abortion or foetal infection. Congenital disease can result in systemic, neurological and progressive eye disease. No vaccine exists for prevention of infection or disease and current drug treatments are not entirely effective.

T. gondii was discovered about 100 years ago in a rodent in North Africa by Nicolle and Manceaux, and later the same year in Brazil as an infection in a rabbit by Splendore. The protozoan was initially thought to be a new species of Leishmania (originally
Leishmania gondii, but it was soon realized to be an entirely new entity). Its name was derived from the Greek words toxon (bow- or crescentshaped) and plasma (cell). Studies into the morphology of T. gondii by electron microscopy began in the 1950s, and the complete life cycle with the identification of the feline family as the definitive host was only elucidated in the 1960s. Hutchison and his team from the University of Strathclyde in Glasgow played a crucial role in the discovery of the sexual cycle occurring in the intestinal tissue of the cat. Since then, T. gondii has become a model parasite to dissect host–pathogen relationships and the immune system.